Point-to-multipoint localized distribution systems are known in the art. Typical systems use multiple low power node antennas, also commonly referred to as cell stations, base stations, and hub stations, which deliver wireless communication services to receiving stations in an area of coverage, e.g., "cell", defined around the node antennas. The node antennas are arranged to form partially overlapping cells. Frequency differentiation, polarization differentiation and similar techniques are used alone or in combination to prevent conflicting signals from adversely affecting communications received by receiving stations at or near areas of overlap between adjacent node antennas and between adjacent sectors of a particular node antenna.
An interesting point-to-multipoint microwave television distribution system utilizes devices in the millimeter wave frequency band, between about 28 GHz and 300 GHz. Of particular interest is the 29 GHz band, from 27.5 to 31.3 GHz, which provides sufficient width to accommodate a number of broadband channels, avoids previously allocated terrestrial bands, avoids satellite down-link bands, permits relatively small sized antennas, and is compatible with known low-cost microwave circuit fabrication techniques.
Each antenna in such a system, or in a similar communication system, is required to provide omnidirectional coverage to form the generally circular cell of transmission coverage around itself. Individual antennas are usually sectored, so that the omnidirectional coverage is achieved by individual directional sector antennas that cover limited azimuth angles. As an example, four sector antennas covering adjacent 90 degree sectors may be combined to form an omnidirectional coverage node antenna.
A disadvantage with typical node sector antennas is their nonuniform gain. Their gain commonly varies as a function of azimuth across the sector and/or as a function of radial distance away from the antenna. Gain is largest near the center of the sector, an area known as the boresight, and gradually lessens at azimuth angles approaching the edges of the sector. As a result, stations disposed in the boresight receive stronger signals than those disposed at azimuth angles approaching the edges of the sector. Similarly, stations disposed at different radial distances see different antenna gain and different path loss. The stations receiving the lower power signals are susceptible to signal outage during fading propagation conditions which are associated, for example, with heavy rainfall. In addition, their signal to noise ratio is diminished, producing a higher potential for other types of signal interference. Other systems use radial antennas, or other types of antennas and share similar problems with receiving stations near adjacent sectors and with receiving stations at different radial distances in the area of antenna coverage.
To compensate for differences in antenna and path gain, prior compensation techniques have varied the power supplied to a specific station. This method works well for terrestrial or satellite point-to-point communication systems where frequency reuse is not locally required and where antennas are separated by large distances. In contrast, it is important to equalize the power spectral density at receiver stations in a point-to-multipoint system so that interference from other node sectors is avoided. For example, when frequency reuse is employed between node sectors, more than one receiving station is on the same frequency at the node, and the level of sector-to-sector interference is increased by having one signal power density higher than the others that share common frequencies.
Thus, there is a need for an improved antenna compensation system for a point-to-multipoint system which addresses the aforementioned difficulties. More specifically, there is a need for an improved antenna compensation system for a point-to-multipoint system which accounts for variable gain across the azimuth range defining a sector while maintaining equalized power spectral density. Because performance may also vary as a function of radial distance from a node antenna, there is a similar need for an improved antenna compensation system for a point-to-multipoint system which accounts for variable gain across radial distance while maintaining the equalized power spectral density.
Accordingly, it is a primary object of the present invention to provide an improved antenna compensation system having a compensated antenna gain pattern and an equalized power spectral density.
Another object is to provide an improved antenna compensation system utilizing multiple forward error correction code rates to compensate for antenna sector pattern gain differences in azimuth so that receiver stations disposed near sector edges have performance equal to that of receiver stations disposed in sector boresights.
Yet another object is to provide an improved antenna compensation system utilizing multiple modulation types having different signal transmission efficiencies to compensate for antenna sector pattern gain differences in azimuth so that receiver stations disposed near sector edges have performance equal to that of receiver stations disposed in sector boresights.
Still another object is to provide an improved antenna compensation system utilizing a combination of multiple forward error correction code rates and multiple modulation types having different signal transmission efficiencies to compensate for antenna sector pattern gain differences in azimuth so that receiver stations disposed near sector edges have performance equal to that of receiver stations disposed in boresights.
An additional object is to provide an improved antenna compensation system individually or severally utilizing variable modulation forward error correction code rates and variable modulation types having different signal transmission efficiencies to compensate for antenna pattern gain differences in radial distance so that receiver stations disposed in radial areas close to the antenna have performance equal to that of receiver stations disposed in radial areas further from the antenna.
A further object of the present invention is to provide an improved analog antenna compensation system in which a signal-to-noise sensitive transmission parameter is varied to compensate for antenna sector pattern gain differences in azimuth and/or antenna pattern gain differences in radial distance to equalize receiver station performance.